1. Field of the Invention
The present invention relates to a composite cord formed of metallic filaments and polymer fiber twisted together, and a pneumatic tire using the composite cord for its reinforcing element.
2. Description of the Background Art
For a reinforcing element of a pneumatic tire, e.g., for its carcass, belt structure, or bead portion reinforcing layer, a metal cord formed by twisting metallic filaments together is often employed. As such a metal cord, a compact cord is used which is made of metallic filaments xe2x80x9cfxe2x80x9d that are filled tight together to allow no gaps between neighboring filaments, of which cross section is shown in FIG. 5. When this compact cord is embedded in rubber to manufacture a reinforcing ply, however, the rubber cannot penetrate into voids created among the filaments. This allows water to enter the voids, so that rust will form therein, which may propagate along the longitudinal direction of the cord. This deteriorates adhesion strength between the cord and the rubber as well as the strength of the cord, thereby causing break of the cord in the tire at running.
In order to solve the problem above, a so-called open cord has been proposed which is made of metallic filaments xe2x80x9cfxe2x80x9d twisted together in such a way that gaps are formed therebetween, of which cross section is shown in FIG. 6.
Another metal cord has also been proposed, of which cross sectional structure is shown in FIG. 7, which is formed by twisting metallic filaments xe2x80x9cf1xe2x80x9d preformed into a three-dimensional spiral shape and not-preformed metallic filaments xe2x80x9cf2xe2x80x9d together to form gaps therebetween. Such gaps around the filaments improve rubber penetration into the interior of the cord.
When manufacturing these metal cords, however, twisting becomes complicated in order to form gaps between metallic filaments, or an additional step of preforming the filaments is required, so that productivity is degraded and the manufacturing cost tends to increase. Further, a preformed metallic filament is relatively poor in strength and rigidity, and its diameter inevitably increases. A ply with such metal cords embedded in rubber is likely to curl up, which makes tire molding difficult.
An object of the present invention is to provide a composite cord that can be fabricated in a simple process at low cost and with high productivity, and is improved in rubber penetration. Another object of the present invention is to provide a pneumatic tire employing the composite cord.
According to an aspect of the present invention, the composite cord has a 1xc3x97n construction (n is an integer from 3 to 12) made of from 2 to 11 metallic filaments and from 1 to 5 polymer fibers having a melting point of from 50xc2x0 C. to 200xc2x0 C. twisted together.
According to another aspect of the present invention, the metallic filament has a diameter of from 0.15 mm to 0.45 mm.
According to a further aspect of the present invention, the polymer fiber is selected from polyethylene fiber and polypropylene fiber.
According to a still further aspect of the present invention, the pneumatic tire employs, for its reinforcing element, a composite cord having a 1xc3x97n construction (n is an integer from 3 to 12) made of from 2 to 11 metallic filaments and from 1 to 5 polymer fibers having a melting point of from 50xc2x0 C. to 200xc2x0 C. twisted together.